A well is typically drilled in order to extract resources from the earth or as a zone to dispose of unwanted materials. The resources may include oil and gas, minerals, or even geothermal heat and things to be disposed may include waste heat, contaminated water, or even carbon dioxide. In many cases the effective production of resources from a well is limited by the permeability of the subterranean formation strata and the total surface area of the well bore and nearby fractures.
The value of oil and gas wells varies greatly according to the rate at which the well produces. Some formations such as shale rock have been found to contain large volumes of natural gas but the permeability of the rock is so low that wells drilled into the shale rock produce only small flows of gas which are insufficient to pay for the expense of the well. To obtain greater recovery of gas, wells are often drilled so that they turn horizontally and run for some distance within a thin shale layer to increase the area of well bore within the layer.
The surface area of the well can be further increased by injecting water into specific areas at high pressure to crack the rock in a process known as hydraulic fracturing. Hydraulic fracturing is the most common technique for increasing the productive surface area of a well. Hydraulic fracturing is not always as effective as desired due to the difficulty of controlling the orientation, thickness, and magnitude of the fractures. Injecting very large volumes of chemically treated water is also an environmental concern as it can potentially pollute fresh water or cause earth tremors. Fractures may also breech and damage natural impermeable barriers underground allowing water to intrude into new areas. In the case of a shale gas well, intruding water can interrupt the production of gas.